1. Field
The present disclosure relates to optical devices. More specifically, the present disclosure relates to optical devices in which a pulse-width modulated signal is used to thermally tune a wavelength-sensitive optical component to a target operating wavelength.
2. Related Art
Wavelength division multiplexing (WDM) can be used to communicate modulated data at different carrier wavelengths on a common optical waveguide. By transmitting data using multiple carrier wavelengths, WDM can overcome optical-fiber congestion, which is a potential problem in optical modules that include parallel optical transceivers with one channel per optical fiber or waveguide. Hence, by significantly reducing the number of optical fibers per optical module, WDM can simplify optical modules and can thereby reducing their cost and size.
One variety of WDM, which is known as “dense WDM” (DWDM) uses a narrow spacing between adjacent wavelengths. In DWDM, data bits are typically modulated directly onto highly stable optical carrier frequencies, and these carriers combined in an optical fiber. DWDM allows a given wavelength band to accommodate a large number of channels, and thus offers high performance. DWDM systems employ a variety of optical devices, such as: optical waveguides, optical modulators, optical multiplexers (such as add filters), optical de-multiplexers (such as drop filters), optical proximity couplers, optical filters, optical switches and optical detectors. While some of these optical devices (such as optical waveguides, optical proximity couplers and optical detectors) are broadband components that are relatively insensitive to ambient temperature changes and process variations, wavelength-selective optical devices (such as resonator-based optical modulators, optical multiplexers, optical filters and optical de-multiplexers), which include wavelength-sensitive optical components, can be very sensitive to these changes and variations.
In principle, optical devices can be made on silicon substrates, because silicon provides many benefits for optical communication. Furthermore, by using silicon-on-insulator (SOI) technology, a silicon optical waveguide can be surrounded by silicon dioxide on all four sides, which facilitates low-loss, on-chip optical waveguides and active devices (such as optical detectors and optical modulators). Silicon-based optical devices can thus be used to implement a wide variety of optical components for use in DWDM communication.
Static errors in manufacturing these silicon-based optical devices include deviations in intended thickness of the silicon layer, deviations in waveguide width, or deviations in the etch depth of ridge waveguides. These deviations cause errors in the operating wavelengths of the wavelength-selective optical devices. For example, a deviation of one nanometer in one of these dimensions can cause 1-2 nm of shift in the operating wavelength of the optical device, depending on its size and other physical technology parameters. It is therefore often necessary to phase tune these optical devices to eliminate these static errors.
Additionally, it may be necessary to phase tune the optical device dynamically (during operation) to maintain accurate alignment of the operating wavelengths (relative to predetermined desired or target operating wavelengths). For example, a tuning precision within a few tens of picometers may be required for a MUX/DEMUX device (and possibly even less in the case of a modulator device), in order to combat dynamic environmental changes such as temperature or material strain. Hence, the ability to tune the resonators both statically and dynamically is paramount in a practical implementation.
Thermal tuning is a useful technique for phase tuning an optical device because it can produce large shifts in the operating wavelength of a silicon optical device. One approach is to put a metal heater on top of the optical device. Another approach heats up the optical device by doping it as a resistor and passing current directly through the device. The former is slightly less efficient than the latter, while the latter must avoid interfering with the operation of the optical device or introducing unwanted optical loss.
Because of the static and dynamic changes in the target operating wavelength, and the high sensitivity of silicon optical devices to temperature, precision control of the heater over a large range is often necessary in order to select the operating wavelength while providing a suitable upper bound on ripple values. For example, if a current source is used to drive between 0-4 mA of current through the heater, up to 4000 steps of current levels may be needed. As shown in FIG. 1, an existing implementation of a thermal-tuning circuit which can achieve such a precise tuning current may include multiple copies of a baseline circuit, each of which can be separately enabled or disabled in order to increase or decrease a reference current or voltage. In particular, a single transistor in this baseline circuit may generate a single step's worth of tuning current, I0. Moreover, generating a current of m·I0 may involve combining the outputs of m transistors. For 1 μA steps and a 4 mA range, 4000 copies of the transistor are needed. However, this may result in a large area overhead.
In addition, because manufacturing variations in the threshold voltage or length may cause each transistor to deviate significantly from I0, which may result in large non-linearities in the total output current, each transistor may need to be trimmed or otherwise compensated to ensure that it truly generates an I0 of 1 μA. Consequently, the existing thermal-tuning circuit may be complicated and expensive, and may have significant power consumption.
Hence, what is needed is an optical device without the above-described problems.